Conventionally, a spark plug is used for igniting an internal combustion engine, such as a gasoline engine (see, for example, Japanese Patent No. 4625531). The spark plug includes a center electrode, an insulator disposed on the outer periphery of the center electrode, a metal shell disposed on the outer periphery of the insulator, and a ground electrode attached to the metal shell and forming a spark gap with the center electrode.
An inspection method for determining whether there is damage in a structural body using an acoustic emission method (which may be hereafter referred to as an “AE method”) is also known (see, for example, Japanese Patent No, 4811932).